So-called "Engineered Footing"

[stochas]

There is a practice among geotechnical engineers in my area that is locally called an “engineered footing.” This practice is used mainly in residential foundation construction where weak soils are encountered at the bottom-of-footing elevation and a firm bearing layer underlies the weak soils. The weak soils are excavated directly underneath the footing down to the firm bearing layer, and the excavation is backfilled with a coarse crushed stone, without compaction. The footing is then poured at the planned elevation directly on top of the crushed stone. I have heard of such installations up to 15 feet deep.

I would like to know if this practice is an appropriate technology for residential foundation construction and whether any research has been published on this topic. Is anyone aware of technical guidance documents that detail basic assumptions and boundary conditions for this type of construction? Also I would like to hear your experience, recommendations, and comments, including any reservations you may have, concerning this practice.

 

[VAD]

Hello stochas:

Yes this seems to be a common practice and while engineered fill can be used re low plasticity compacetd clay to repace the soft or weak excavated ground the use of graular material has many obvious advantages in the relation to speed and eliminating compaction issues. Site constraint s my promote this method as well. As the loads are often light minimal or no compaction is often used but one has to be careful not to take this to extreme.

One may be concerned about this approach creating a water trap below the footing if this system is not connected to the perimeter drainage of the house etc. It punctuates if the area is subject to freezing temperatures and granular is not clean so as to prevent frost heaving. Sometimes we use low cost graular with appreciabl;e fines which is a frst susceptible material

one of the things we must appreciate is the performances of homes subjected to this type of construction since while we can have differring opinions somethings perform differently than our expectations based on site specific conditions. This must be looked at carefully in our evaluation. However very often we accept a practice from visual observations but cannot provide reasons simply because to get rational answers we may have to undertake fundamental research on materials etc which is not the norm except for a government agency.

You may find some information in the Building Research Digests of Canada 1965 vintage which are incorporated in Canadian Building Codes as well to some extent.

Depending on the size of structure and its use the services of structural and geotechnical engineers are not usually warranted and for such then the hands on trades people take the lead. When things go wrong however, the Engineer is often required to find the answers as we have seen in many of the questions raised on this forum. To have an engineer involved is thought to be too costly by builders and clients.

If one is required to provide recommendations for such structures it is my opinion that one understands why a particular approach is used so that you are able to defend yourself should something go wrong. It is not satisfactory to say that you did not know the reason and is standard practice.

I may have digressed from the main thrust of the question.

In summary some practices may yield perfectly good performances but engineers must be able to understand why.

Hope this helps.